1. Technical Field
This invention relates to lead-free, high-octane gasolines for use in motor vehicles and more particularly to such a gasoline having unique compositional and physical characteristics.
2. Prior Art
It is known that among various qualities of motor gasoline, the vapor pressure is important to ensure efficient operation of automobile engines. This pressure if at too low a level renders the engine less startable at low temperature and if at too high a level leads to marred driveability at elevated temperature. To cope with this sort of dilemma, the volatility parameter is necessarily adjusted to match those climatic conditions in which the gasoline is used. As concerns Japan's seasonal changes of climate, such parameter is desired to be lower in summer season from May through September and conversely higher in winter season from October through April.
In 1983 a lead-free, high-octane gasoline came into the market which had a research octane number of 98, and in 1987 another commercial grade made its debut with a similar number of 100. Due to their freedom from blending of a methyl-t-butylether compound (MTBE) which appears hereunder in detail, both grades rely solely upon their respective hydrocarbon components in gaining the above octane ratings and hence have a large proportion of aromatic hydrocarbons. As disclosed for instance in U.S. Pat. No. 4,824,552, a further grade has been proposed which is formulated to have certain distillation and composition properties, thereby improving acceleration and other important fuel qualities.
MTBE is commonly accepted for use as a component in gasolines of high octane requirements. In the United States automobile industry, such compound is reputed for contributing to reduced amounts of carbon monoxides and hydrocarbon components escaping as undesirable exhaust emissions. MTBE-blended, high-octane gasolines are disclosed in Japanese Patent Publications Nos. 50-35524 and 60-11958. In 1991 this type of gasoline was marketed in Japan.
Though large in octane number, MTBE is relatively low-boiling and light-natured. This means that blending of MTBE will produce a gasoline of a light nature even with a high octane requirement. While satisfactory startability of a cold engine can be expected with use of light gasoline, MTBE blending is reported susceptible to poor engine startup. Another but serious problem is that MTBE tends to increase nitrogen oxides (NOx) in exhaust gas.